The present invention relates broadly to a process for wet grinding of natural and synthetic carbonates, as, for example, calcium carbonate in wet suspension systems.
The milling of calcium carbonate in the mineral processing industry is steadily increasing to produce, for instance, fillers suitable for paper filling and coating. In practice of this technology fine, fractured calcium carbonate particles are manufactured by specially developing grinding mills in order to obtain composite particles containing 90% below 2 micron diameter sizes. These mills grind the calcium carbonate in slurry form generally containing about 70% by weight of solid content.
This high solids content is necessary in order to get good grinding performance and is also the result of consumer preferences with respect to many applications for the finished product. However, the viscosity of the slurry rises along with increased solids content. This increase in viscosity increases the energy requirement for the grinding process and causes other problems.
In order to facilitate the grinding it has been customary and necessary to add certain amounts of chemical dispersants, for example, those based on polyacrylates, polyphosphates or polycarbonic acids. These chemical dispersants favorably decrease the viscosity of the slurry but, unfortunately, do so only to a limited extent. Although the viscosity initially decreases upon addition of the dispersant, a point is reached beyond which increased amounts of dispersant no longer reduce the viscosity of the slurry but actually causes it to rise. A further disadvantage in using higher amounts of dispersants in the process is that the dispersants promote the growth of bacteria which can result in spoilage of the slurry. Biocides may then be required to control growth of the bacteria. High concentration of dispersants decrease the retention of the carbonate when used as a filler in paper products manufacturing and thus increases the need for using retention agents in the paper manufacturing process. The waste water developed during the process also becomes contaminated to an increased extent when greater amounts of dispersants are used. The present invention provides solutions to the above mineral milling processes as noted in the succeeding section.